Thermochromic materials exhibit changes in physical properties such as electrical conductivity, absorption, reflectance and refractive index as the result of thermodynamic state changes between the semiconductor and metal state. Thermochromic materials, such as certain vanadium oxides and titanium oxides, have relatively low absorption in the semiconductor state and high absorption and high reflectance in the metal state.
The thermodynamic transitions between the semiconductor and metal states are reversible and take place sufficiently rapidly that thin films of such materials are feasible as optical modulators. For example, U.S. Pat. No. 3,664,726 to Mattis, U.S. Pat. No. 3,484,722 to Barker and U.S. Pat. No. 3,497,286 to Morton disclose devices in which such materials are used for modulating light. However, none of these devices exploit both the high transmission characteristics of these materials in the semiconductor state and the high reflectance properties of these materials in the metal state. Exploitation of both properties by the present invention makes it possible to use these materials in high speed beam splitters, which have applications in the switching between optical fibers, the tapping of optical fibers, and in a transmit-receive switch for a laser radar.